The present invention relates to a suspension load beam in a disc drive, such as a hard drive using a magnetic storage medium. More particularly, the present invention relates to a disc drive suspension load beam using a damping material to reduce high frequency vibration.
Disc drives are one of the key components to store data in a computer system. In a basic hard disc drive, data is stored in a magnetic medium formed on a surface of a rotating disc. The hard disc drive reads and writes information stored on tracks on a disc bearing the magnetic medium. To do this, a read/write head that includes a transducer carried by a slider assembly is placed in close proximity to the surface of the magnetic medium. The slider is attached through a gimbal system to a distal end of suspension load beam which includes the suspension load beam. The proximal end of the suspension load beam is attached to an actuator arm which is rotatably controlled by a voice coil motor (VCM). The disc drive system sends control signals to the voice coil motor to move the actuator arm and the suspension supporting the read/write head across the disc in a radial direction to the target track. The positioning of the read/write head over the magnetic medium is controlled by a closed loop circuit for better accuracy. In addition to the active controlling signal from the closed loop circuit, the precise positioning of the read/write head is affected by a dynamic balance between two vertical forces. The first force is a gram load applied by the suspension load beam to bias the head toward the disc surface. The second force is an air bearing lifting force caused by the fast motion between the slider and the disc surface. Roughly, the looped control system controls tracking (i.e., radial positioning of the head) while the dynamic balance determines fly-height (i.e., head-media spacing). However, as the areal density of concentric data tracks on magnetic discs continues to increase (that is, the size of data tracks and radial spacing between data tracks decrease), hard disc systems also use active control for more precise vertical positioning of the head.
One of the most significant adversarial conditions affecting precise positioning of the read/write head in a disc drive system is vibration, particularly that caused by head suspension resonance. Many types of vibration exist in a disc drive system to cause fluctuation of the magnetic read/write head positioning. However, vibrations that occur at frequencies far away from a resonant mode (e.g., less than one third of the first resonant mode) are usually less serious concerns. In contrast, vibrations that cause resonance of the system are often much more serious obstacles in improving areal density and rotation speed of the disc drive system. Every closed loop servomotor system has a predetermined bandwidth in which resonances occurring within the bandwidth degrade the performance of the servomotor system. In a hard disc drive system, for example, windage (fluid turbulence caused by airflow) and head vibration occur at a frequency close to a resonant mode of the suspension load beam and thus cause the suspension head assembly to resonate at large amplitudes. Windage and head vibration, however, are not the only sources that cause resonance in a hard disc drive system. In today's high-speed hard disc drives, the servomotor that moves the parts at high frequency may also cause resonance. In addition, when it is desired to position the magnetic head and a specific track location, the voice coil motor is driven by a voltage that has a very short rise time to accelerate the actuator very quickly. Once the actuator is in motion, the voltage levels off and the actuator approaches a constant velocity. As the actuator approaches the target location on the disc, a similar, but inverse abrupt voltage pattern is applied to the voice coil motor to stop the suspension actuator. This sequence of voltage change is best represented by a square wave, which is a superposition of many waves of different frequencies, according to Fourier transform. The operation of the servo system in a hard disc drive to move the suspension head assembly thus has inherent frequency components that may excite resonance.
Resonance degrades the performance of a disc drive in several ways. First, severe resonance, especially that of torsion or sway mode, may cause the magnetic read/write head to move away from the target track and thus result in data reading/writing error. Second, resonance in the vertical direction, such as that caused by resonance in bending mode, may cause fluctuations in the fly height of the read/write head to result in data error as well. In extreme cases, vertical fluctuations may even cause catastrophic damage of the disc drive due to direct contact between the head and the disc surface. Third, during resonance, the transducer element of the read/write head is forced to modulate, causing a significant decrease in the signal to noise ratio of the system and increase of the non-repeatable run-out (NRRO).
Significant efforts have been made to alleviate the problem of resonance. Various methods have been used. Optimization of the system is essentially a balance of several factors, often gaining on one aspect at a cost of sacrificing another, as commonly found for a spring-mass-damper system. A suspension load beam must be sufficiently stiff in order to be mechanically and the structurally stable. Unstable materials suffer change of physical dimension with time, so-called cold flow or creep. To maintain a sufficient stiffness of the suspension load beam, a stiff metal piece, such as stainless steel sheet material, is used to make at least part of the suspension load beam. In principle, stainless steel part of the load beams could be made thicker to increase the bending and torsion mode frequencies, but the greater mass significantly degrades the performance of the actuator assembly by increasing the inertia of the arm. An increased inertia will decrease the access time to position between data tracks and increase the current requirements necessary to move the voice coil motor and the suspension head assembly. These changes then cause other problems such as increased heat within the disc enclosure and increased power requirements. A thicker steel arm will also result in a higher mass assembly that will cause significant degradation of shock resistance of the disc drive system. Higher mass also leads to lowest stability. Although materials having higher stiffness/mass ratio than that of stainless steel do exist and have been experimented, solutions of this type have not become widely acceptable mainly due to high cost and low reliability issues. Other methods for increasing the stiffness of the suspension load beam without increasing the mass or switching to a more expensive material are also suggested. U.S. Pat. No. 5,408,372 to Karam, for example, uses a micro-stiffening technique to control resonance in the suspension system of a disc drive.
Another approach to reduce resonance of the suspension head system in a disc drive is to use dampers. U.S. Pat. No. 3,725,884 to Garfien shows a support arm on which the magnetic head is supported by a spring member and up and down motion of the magnetic head is damped by an additional leaf spring in rubbing contact with friction pads. U.S. Pat. No. 4,760,478 to Pal et al. uses a layer of damping material fixed to the top of the elongated flat load beam and a constraining member fixed in contact with the damping material to reduce the resonance. U.S. Pat. No. 6,297,933 to Khan et al. discloses a disc drive suspension load beam having a damping structure containing an organic damping material. The damping structure is attached in a load beam recess sized and shaped to limit exposure of organic damping material to the ambient atmosphere.
Dampers commonly used are damping structures attached to the suspension load beam. Dampers are believed to absorb vibration energy when repetitive deformation (caused by vibration) of a material is dissipated through internal energy losses, usually in the form of heat. One form of internal energy dissipation is through shear energy absorption in the layer of damping material. It has been known that materials that exhibit a large ratio of dynamic loss moduli to dynamic storage moduli, tan δ, tend to have high shear energy absorption and thus are good candidates for making dampers. An exemplary type of materials exhibit a large tan δ is viscoelastic materials, which when deformed, have a stress proportional to both the deformation and the rate of deformation. Viscoelastic materials also exhibit creep and relaxation behavior. Creep means that under constant stress the deformation increases in time. Relaxation means that under constant fixed deformation the stress decreases steadily in time. These properties generally relate closely to damping properties because they are opposite to that of a spring material which is known to preserve dynamic energy during motions without converting the energy into thermal energy.
A number of approaches have been taken to achieve material properties sufficient for damping purposes. Specialized formulations of cross-linking polymers have been developed which exhibit damping in specific applications. Epoxy formulations have been developed for damping vibrations in magnetic read/write heads, as disclosed in U.S. Pat. No. 5,270,888. Acrylic copolymers for damping are commercially available in, for example, sheeting form. Silicone chemistries have been developed for damping, as disclosed in U.S. Pat. No. 5,434,214 to Sutton et al. In addition, a composite damping material is suggested in U.S. Pat. No. 5,965,249, in which a highly viscous damping fluid is entrapped within the pores of a porous material (such as an expanded polymer, felt material, foam, fabric, metal, etc.). The patent further suggests attaching a piece of the composite damping material to a surface of a mechanical member in a disc drive to reduce vibration.
As suggested in U.S. Pat. No. 5,965,249, however, dampers used for reducing suspension head resonance are conventionally affixed separately on a surface (e.g., top surface) of the suspension load beam, typical using an adhesive, instead of being formed as an integral part of the suspension load beam. This is in line with the conventional concept of suspension damping in which the stainless steel part of the beam is considered the base structure to provide stiffness and mechanical integrity, while an add-on damper is considered to provide damping only. To achieve this goal, much effort has been made to provide a damper that does not cause significant structural mortification of the base structure.
In order to maximize the shear energy absorption in the damping material, elaborate designs of using a constraining member have been proposed. For example, U.S. Pat. No. 5,594,607 to Erpelding et al. discloses a laminated suspension having a stainless steel stiffener layer, a top constraining layer (which also functions as a conductor layer), and a viscoelastic dielectric damping layer, wherein the constraining layer has a pattern of land areas etched thereon to increase the shear energy absorption in the damping layer.
Using a different approach, U.S. Pat. No. 5,187,625 to Blaeser et al. proposes a head suspension load beam which incorporates a layer of damping material throughout the entire structure of the suspension to reduce the amplitude of all resonant modes of vibration. It is believed that because the point on the suspension structure at which maximum strain energy occurs may change for each mode of vibration, it is advantageous to distribute the damper throughout the entire structure in order to cover all possible vibration modes. However, the use of the damping material layer throughout the entire suspension structure is still in line with the conventional concept of suspension damping in which the base structure of stainless steel provides stiffness and mechanical integrity while the add-on damper provides damping.
In yet another different approach, alloys having high intrinsic damping properties have been proposed to replace the conventional stainless steel to make suspension load beams in a disc drive. An example of such alloys is found in U.S. Pat. No. 6,361,740.
At the same time, with the increasing demand for disc drives that are more reliable, quieter and faster, and have larger storage capacity (with increased areal density) and sometimes smaller overall disc size, there is an increasing need for a disc drive suspension system having better balanced optimization between several performance properties including damping property, stiffness and the structural integrity.